Oct. 8, 2010 -- Low testosterone levels in older men with memory problems may signal progression to Alzheimer's disease or even increase the risk for developing age-related dementia.

In a newly published study, older Chinese men with early memory declines who did not yet have Alzheimer's were far more likely to develop the disease over a year of follow-up if they had low testosterone at enrollment.

The study was small, but the findings suggest low testosterone may be an independent predictor of rapid cognitive decline in older men with early memory loss, Saint Louis University Medical Center professor of gerontology John Morley, MD, tells WebMD.

"The next step would be to replace testosterone in these men with memory declines to see if we could slow the progression to Alzheimer's disease," Morley says.

The study, led by researchers at the University of Hong Kong, included 153 older Chinese men followed for one year.

All the men underwent testing to assess memory function at enrollment, and 47 were determined to have evidence of mild cognitive impairment.

Over the course of the next year, 10 men received a diagnosis of Alzheimer's disease. All were in the previously identified group with early memory declines and all had low levels of free testosterone in blood samples.

Free testosterone level was one of only three independent predictors of progression to Alzheimer's disease in the study. The others were high systolic blood pressure and presence of the apolipoprotein E (ApoE) protein genotype, which is an established genetic risk factor for Alzheimer's disease.

According to the National Institute on Aging, the genetic variation occurs in about 25% of the population and about 40% of people who develop Alzheimer's disease late in life.

Men in the study with the genotype were five times as likely to develop Alzheimer's disease over the next year as men without it.

The research appears in the online version of the Journal of Alzheimer's Disease.

The study is not the first to suggest a protective role for testosterone in Alzheimer's disease.

In 2004, investigators at Wayne State University examined testosterone levels in men enrolled in a larger aging study. They found that every 50% increase in free testosterone in the bloodstream was associated with a 26% decrease in the risk of developing Alzheimer's.

Men who went on to develop Alzheimer's disease had about half the free testosterone in their bloodstreams as men who did not.

While the research suggests a role for testosterone in the prevention of Alzheimer's disease, study researcher Scott Moffat, PhD, says it is too soon to recommend testosterone treatment for men at risk for cognitive decline.

"It is not really clear if testosterone is protecting the men in these studies or if levels are reflective of some other factor, such as overall better health," he tells WebMD.

Even if testosterone were proven to help protect the brain, the benefits of testosterone treatment would have to be weighed against the risks, he adds.

"We know that testosterone promotes the growth of prostate tumors, and that is why men who are prescribed it need to be monitored very closely," he says.